1. The Field of the Invention
The invention is in the field of dental restoration compositions and methods, more particularly in the field of compositions and methods for bonding an amalgam restorative to a dental substrate. The bonding compositions are cured in steps to promote enhanced adhesion of the amalgam to the dental substrate.
2. The Relevant Technology
In the field of dental restorations, a variety of materials have been used to fill and restore cavities and other defects in a person""s teeth. These include metals, such as gold, silver, platinum, alloy, and amalgams, curable polymers such as polyalkyl methacrylates, polycarbonates, cured products of polyfunctional vinyl monomers, composite resins comprising fillers and the aforementioned polymers, and ceramics such as porcelain. Because such materials do not readily adhere to dental tissues, the tooth, and in particular the pulp, should be sealed or capped in order for the tooth to remain vital and avoid being infected with bacteria.
Polymeric restoration materials such as composites, as opposed to metal-based restoration materials such as amalgam, are fairly compatible with bonding agents and adhesives. This quality allows composites and other polymeric restoration materials to be directly bonded to the teeth, thereby allowing such materials to be firmly and sealingly attached to the patient""s tooth, regardless of the shape of the hollow or dental preparation formed in the tooth. This allows the dental practitioner to remove only so much of the decayed or damaged tooth as will prevent further damage or decay to the tooth once the dental preparation has been sealed and filled using a combination of the composite restoration material and bonding agent. In addition, composite restoratives can be formulated to match the color of the patient""s teeth, thus resulting in a more cosmetically pleasing tooth restoration compared to amalgam or other metal restoratives, which typically assume the color of the metal or metals that make up the restorative.
Amalgams and other metal restoratives are generally incapable of forming a strong bond with dental substrates, even when used in combination with conventional adhesive bonding agents. Restoration techniques that involve the use of amalgam or other metal restoratives typically require the removal of much larger quantities of the dental substrate, as compared to composite restorative techniques, in order to provide for mechanical overhangs of dental tissue that mechanically retain the cured or hardened metal restoration material within the dental preparation. Thus, in contrast to restoring a tooth using a polymer-based restoration materials, the practitioner must not only remove the decayed or damaged portion of the tooth but also so much of the surrounding healthy and undamaged tooth as will result in a dental preparation of the proper shape for mechanically retaining the hardened or cured metal restoration material. In general, dental preparations suitable for receiving amalgam or other metal restoratives advantageously widen toward the interior of the tooth. In this way, the hardened or cured metal restorative is mechanically held in place by overhanging or converging dental tissues. Because there is no significant bond between the amalgam and the tooth, microleakage at the margins can occur.
Even though amalgams and other metal restoration materials are typically stronger and more durable than polymeric restoration materials, the use of amalgam and other metals generally results in a weaker, less durable restored tooth compared to a tooth restored using a composite restorative. Increased tooth weakness results from the necessity of removing substantially more of the tooth than simply the decayed portion and from the inability of the amalgam or other metal to strongly bond to the tooth. Preparing the tooth to receive the amalgam or other metal restorative results in a prepared tooth that is at once smaller in size and which contains a much larger weakening discontinuity or void therethrough compared to a tooth prepared to receive a polymeric restorative. In addition, the lack of any significant bond between the remaining dental tissue and the amalgam or other metal restorative results in much lower total composite strength of the finished tooth compared to a restored tooth in which the restorative and tooth form a strong composite bond.
In view of the advantages of composite restorative materials and techniques described herein, the use composites is rapidly increasing, at least in the United States, while the use of amalgams or other metal restoratives is generally declining. Nevertheless, there is still significant demand for amalgam restoratives. Many dentists have used amalgam restoratives for decades and are very skilled in the techniques used to prepare teeth to receive such restoratives. Dentistry, like other professions, is a skilled profession in which practitioners often opt to continue using procedures that they are comfortable with. Similarly, many patients who have had amalgam fillings all their lives are conservative and resistant to change simply because composites are new and possibly xe2x80x9chigh techxe2x80x9d. As the common adage goes, xe2x80x9cif it ain""t broke, don""t fix it.xe2x80x9d
Moreover, amalgam restoratives have certain advantages over composite restoratives, not the least of which is cost. Another is ease of use. Amalgam restoratives typically comprise a mixture of relatively inexpensive metals (e.g., mercury, silver, copper and tin) that, when mixed together, are initially pliable and packable but which quickly cure or harden into a durable tooth filling as the metals react and become compounded. Examples of commonly-used amalgam restorative materials include TYTIN, which is manufactured and sold by Kerr Corporation, located in Orange, Calif. and VALIANT PH.D., which is distributed in the United States by Vivadent/Ivoclar North America, located in Amherst, N.Y. In contrast, composite restoratives typically include polymerizable resins, fillers, adhesives, and curing agents which, in the aggregate, are many times more expensive than amalgam restoratives. Whereas the cost differential may not matter to the wealthy or vain who will pay anything for stronger and/or more cosmetically appealing teeth, it does matter to many, particularly individuals on limited budgets and in less affluent countries.
Another problem with composite restoratives is that they are subject to greater variability in quality and strength compared to amalgam restoratives. Their use also requires more technical skill. When polymers cure they tend to shrink, and polymerization shrinkage is a problem that is generally dealt with by technique rather than by formulation. Technique is a learned trait, and a dentist with poor technique can improperly bond the composite to the patient""s tooth. A composite restorative that is not adequately bonded to the tooth, i.e., that is partially detached, can provide ingress of bacteria into the dentine or pulp. Such restorations should be repaired by removing and replacing the improperly bonded composite material. Worse, improper placement of a shrinkable composite within a fragile tooth can cause it to crack or fracture, thus requiring major dental repair such as a crown.
In view of the foregoing, the choice between whether to use composite restoratives, on the one hand, or amalgam restoratives, on the other, to repair a decayed or damaged tooth comes down to weighing the respective advantages and disadvantages of each and then determining which are most important to the dental practitioner and/or the patient. When properly placed, the use of composite restoratives generally result in a much better composite bond with the tooth and a more cosmetically pleasing look. On the other hand, amalgam restoratives provide greater simplicity of use and much lower cost. For this reason, restorations involving a child""s xe2x80x9cbabyxe2x80x9d (or milk) teeth are performed using amalgam, since such restorations are, by definition, only temporary. Nevertheless, as stated above, amalgam restoratives are unable to form strong bonds with dental substrates such that the dental preparation must be large enough and properly shaped in order to provide for mechanical retention of the hardened amalgam.
Some attempts have been made to bond amalgam to dentin and other dental tissues, but with little success at yielding a commercially viable or professionally acceptable solution. Hence, the vast majority of dental restorations involving the use of amalgam restoratives are performed without the use of an adhesive bonding agent.
In view of the foregoing, it would be an advancement in the art of dental restorations to provide adhesive compositions and methods that provided for strong and reliable bonding between amalgam restorative materials and dental substrates. It would be a further advancement if such adhesive compositions and methods were able to seal the underlying dental substrate, as well as provide an improved seal between the amalgam restorative and the dental substrate.
Such compositions and methods for adhering an amalgam-based restorative material to a dental substrate are disclosed and claimed herein.
The present invention encompasses greatly improved bonding compositions and methods for restoring a patient""s tooth using an amalgam restorative material. More particularly, the compositions and methods according to the invention allow the dental practitioner to form a much stronger bond between a dental substrate and an amalgam restorative material. This, in turn, yields a stronger and more durable tooth restoration compared to conventional amalgam-based restorations due to greatly increased composite strength. In some cases, the dental practitioner may be able to remove less dental material from the decayed or damaged tooth that would be otherwise be required if mechanical retention were the only force holding the tooth and the amalgam restoration together. Keeping more of the original tooth intact will generally preserve more of its strength and reduce the risk of subsequent breakage or cracking of the restored tooth. Such compositions and methods also advantageously seal the underlying dental substrate, including the dentin tubules, and result in a substantially better seal between the substrate and amalgam, either of which would be expected to reduce tooth sensitivity and the propensity of the tooth to become decayed in the future. Such bonding greatly reduces or eliminates microleakage at the margins.
The amalgam bonding compositions according to the invention are formulated so as cure or harden in steps. In particular, they are formulated to only partially cure within the dental preparation prior to packing the amalgam into the dental preparation. The partially cured bonding composition remains pliable and deformable, particularly at the surface, for reasons that will be discussed more fully below. This permits the upper layer of the bonding composition to become physically disrupted when packing the amalgam within the dental preparation, which substantially increases the mechanical interaction and interface area between the bonding composition and the amalgam. After the amalgam restorative material and bonding composition have both been cured, this physical disruption of the upper layer of the bonding composition and resulting increased mechanical interaction and interface area between the two results in greatly increased bond strength and adhesion between the bonding composition, amalgam restorative, and the dental substrate.
The bonding compositions according to the invention preferably include at least one polymerizable material, a first polymerization initiator that allows the dental practitioner to partially polymerize or cure the polymerizable material within the dental preparation prior to packing the uncured amalgam restoration material therewithin, and a second polymerization initiator that causes the polymerizable material to continue to polymerize or cure beneath the packed amalgam restoration material. These work together to form a strong bond between the amalgam, bonding agent, and dental substrate. The inventive bonding compositions may include fillers, active agents, adjuvents and other additives as desired in addition to the components specifically identified herein. They may be one-part or multi-part bonding systems.
The polymerizable material may include one or more polymerizable promoters, such as methacrylic acid or derivatives thereof In addition to, or instead of the polymerizable promoter, the polymerizable material may include one or more polymerizable resins, such as 2-hydroxyethyl methacrylate (HEMA) or bis glycerol methacrylate phosphate. One of ordinary skill will readily understood, when reading the present disclosure, that any appropriate polymerizable material may be used so long as the resulting bonding composition has desired properties and provides desired bonding characteristics between the dental substrate and amalgam restorative.
The first polymerization initiator preferably comprises at least one photoinitiator, examples of which include phosphine oxide, phenyl bis(2,4,6-trimethyl benzoyl); 2-hydroxy-2-methyl-1-phenyl-1-propanone; and camphoroquinone. The identity and concentration of the first polymerization initiator are selected so that the polymerizable material within the bonding composition is only partially polymerized upon placing the bonding composition within a dental preparation and irradiating the composition with radiant energy. Because the aforementioned photoinitiators are consumed during polymerization, i.e., they are reactants rather than catalysts, their concentration can be selected so as to result in a desired degree of partial polymerization. Thus, if an amount X of the photoinitiator is required to fully polymerize an amount Y of the polymerizable material, including a quantity of the photoinitiator that is less than X would be expected to result in only partial polymerization or curing of the polymerizable material. One of ordinary skill in the art will, through routine testing, be able select an appropriate quantity of photoinitiator that will result in a desired level of partial polymerization of a given quantity of polymerizable material. In this way, the level or degree of partial polymerization can be controlled.
The second polymerization initiator preferably comprises at least one chemical initiator that is able to cause the polymerizable material to continue to polymerize or cure after placement of the amalgam restorative within the dental preparation. Because amalgam restoratives are opaque, they generally do not permit further polymerization of the underlying bonding composition by photoinitiation, i.e., they substantially or entirely shield the bonding composition from all light that may be emitted by a dental curing light. For this reason, the second polymerization initiator will typically be a chemical initiator that causes polymerization of the polymerizable material in the absence of radiant energy. An example of an appropriate chemical initiator is benzoyl peroxide. The second polymerization initiator not only causes the polymerizable material to more completely or entirely polymerize, it also allows the dental practitioner to time the curing or hardening of the bonding agent so that it remains pliable while packing the amalgam into the dental preparation and then cures into a hardened and less-pliable material after the amalgam has been packed.
One of ordinary skill will readily appreciate that timing of the extent of final cure of the bonding agent may be carried in any desired manner. In a preferred embodiment, the amalgam itself can be used to trigger the final curing or polymerization of the polymerizable material within the bonding agent. Benzoyl peroxide is relatively stable in the presence of many polymerizable materials and photoinitiators, even after partially polymerizing the polymerizable material by irradiation with radiant energy from a dental curing light. This allows the chemical curing process to be controlled or prevented prior to packing the amalgam. In some cases, the benzoyl peroxide is sufficiently stable such that the bonding composition can be premixed as a stable, one-part composition. Keeping the one-part composition from overheating helps to prevent it from prematurely polymerizing. However, when the bonding composition comes into contact with the amalgam restorative, particularly when the upper layer of the partially cured composition is disrupted and agitated during packing of the amalgam restorative into the dental preparation, the interaction of metal within the amalgam and the benzoyl peroxide causes the benzoyl peroxide to decompose. This, in turn, causes the polymerizable material to further polymerize so as to cause the bonding agent to harden or cure. Such interaction may involve a catalytic reaction, heat or both.
Without being limited to any particular theory, it is believed that exposure of the bonding agent to air inhibits polymerization of the polymerizable material. As a result, after the bonding agent has been applied to a dental substrate and then exposed to radiant energy (e.g., from a dental curing light) to induce partial polymerization of the polymerizable material, the region of the bonding agent that is shielded from the air is preferentially cured or hardened compared to the region that is exposed to the air. Thus, when partially curing the bonding agent with the curing light, the underlying region or layer of the bonding agent adjacent to the dental substrate is preferentially cured or hardened, while the upper region or layer of the bonding agent at the exposed surface forms an xe2x80x9cinhibition layerxe2x80x9d (or xe2x80x9cinhibited layerxe2x80x9d) of less cured or hardened bonding agent. This differential in curing provides the benefit of allowing the upper surface layer to remain pliable and easily deformable so that, when the amalgam restorative is packed into the dental preparation, the upper portion of the bonding agent can beneficially become disrupted and agitated so as to increase the interface area and mechanical interaction between the bonding agent and amalgam. On the other hand, the more fully cured or hardened bonding agent adjacent to the dental substrate strengthens the bond between the bonding agent and the dental substrate. This helps to prevent separation of the bonding agent from the dental substrate while packing the amalgam into the dental preparation.
After the amalgam has been packed into the dental preparation and placed over the bonding agent, the chemical initiator causes the polymerizable material within the bonding agent to continue curing and hardening, including the initially less cured inhibition layer. In the case of benzoyl peroxide, it is believed that this additional polymerization is triggered by some interaction between the benzoyl peroxide and the amalgam that causes the benzoyl peroxide to decompose and catalyze further polymerization. Shielding of the bonding agent from air by the packed amalgam restorative facilitates curing as the air that formerly inhibited polymerization is displaced from within the filled tooth by the amalgam.
In a preferred method of restoring a tooth, the decayed or damaged portion of the tooth is removed, together with additional dental tissue as necessary to form an appropriate dental preparation or hollow. In many cases, it will be possible for the dental preparation to be significantly smaller than conventional dental preparations used in conventional amalgam restoration procedures, e.g., formed without overhanging dental tissue designed to mechanically retain the amalgam within the dental preparation, as is generally required using convention amalgam restoration methods. The dental preparation is preferably conditioned with an aqueous acid solution, such as a 35% phosphoric solution, in order to etch the enamel and/or remove the smear layer that typically forms when removing dentin by drilling or abrading. Thereafter, the dental preparation is preferably washed with water to remove the acid and any dissolved minerals and loose, detached dentinal materials. After conditioning and washing, the tooth surface may be dried with pressurized air and/or suction.
The conditioned, washed and dried dental preparation is then preferably coated with an amalgam bonding composition according to the invention. In particular, the bottom, sides and any other surface of the dental preparation that will contact the amalgam restorative is coated with the bonding composition. The bonding composition is then partially cured or polymerized. This is preferably accomplished by light curing a composition that has been formulated to include enough photoinitiator to only partially cure the composition to a desired extent, e.g., so as to cause the layer of bonding composition adjacent to the dental substrate to preferentially cure sufficiently so as to form a bond between the dental substrate and the bonding composition, while leaving the exposed upper layer less cured and more pliable. This is the so-called xe2x80x9cinhibitionxe2x80x9d or xe2x80x9cinhibitedxe2x80x9d layer. In alternative embodiments, the degree of polymerization may be controlled in other ways, such as by irradiating the bonding composition for less time than is necessary for it to more completely cure, or by including a slow acting chemical initiator (e.g., in a two-part system).
After the bonding agent has been partially cured so as to form a bond with the dental substrate, while remaining pliable or deformable in the region of the exposed surface, an appropriate uncured amalgam restorative material is packed (or xe2x80x9ccondensedxe2x80x9d) into the dental preparation. The amalgam is packed into the dental preparation using conventional packing tools known in the art. As the packing tool pushes the amalgam into the dental preparation, typically with short, but firm, downward thrusts toward and through the amalgam, the deformable upper region of the bonding composition is deformed and disrupted, thereby causing troughs, peaks, and other significant irregularities to form in the surface of the bonding composition. Fingers of the deformable bonding agent can extend into the uncured amalgam so as to form roots therein. The pliable, uncured amalgam restoration material displaces the air and assumes a conformation that is complementary to the irregularities in the disrupted bonding composition. This greatly increases the mechanical interaction between the bonding layer and the amalgam. On the other hand, the region of the bonding agent adjacent to the dental substrate is preferably sufficiently cured so as to resist deformation and detachment from the dental substrate.
Once the amalgam has been packed and shaped, it is allowed to cure. Conventional amalgams cure over time after initially mixing the initially separate metals together. Over time, the metals react and become bonded, which causes the amalgam restoration material to become hardened. At the same time, the amalgam also preferably causes one or more chemical initiators to begin the final polymerization of the polymerizable material in order to finally cure or harden the bonding agent. Whereas the two curing reactions may coincide to some extent, they may differ in duration. In alternative embodiments, one or more chemical curing agents can be selected, stored separately, and then mixed with the other components of the bonding composition in order to trigger the second curing step independently from contacting the amalgam with the bonding agent.
The various embodiments of the bonding compositions and methods according to the invention provide a number of advantages over the prior art. For example, the increase in bond strength between the dental substrate and amalgam restorative provides for greater composite action between the tooth and the amalgam. Reliable bond strengths between about 25-37 MPa have been achieved when using the most preferred compositions and methods described herein. This should, in and of itself, yield a stronger restored tooth. Moreover, greatly increasing the bond strength between the amalgam restorative and the dental substrate should, at least in theory, reduce or eliminate the need for mechanical retention of the amalgam restorative within the dental restoration. This would potentially allow the dental practitioner to remove significantly less tooth material when forming a dental preparation for filling with an amalgam restorative. Improving the bond between amalgam restorative materials and dental substrates would also be expected to improve the seal between the amalgam restorative and the tooth, thereby greatly reducing the chance that bacteria could enter and fester within gaps or fissures between the amalgam and the tooth.
These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.